Heat treatment is used in the food processing industry to eliminate pathogens and for other purposes. For example, milk may be heated to about 145° F. for about thirty minutes, or to about 162° F. for about fifteen seconds to destroy or deactivate disease-causing microorganisms found in milk. These heat treatment processes are commonly referred to as pasteurization. Milk or cream may also be treated by heating to 280° F. to 302° F. for two or six seconds (or more) in a process referred to as ultra-high-temperature (“UHT”) pasteurization. Pasteurization and UHT pasteurization may not entirely sterilize the product being treated, but may be effective for killing or deactivating pathogens present in the product.
Heat treatment of liquid or otherwise pumpable materials like milk and cream may be indirect or direct. In indirect heat treatment systems, the heating medium remains separate from the foodstuff and heat is transferred to the foodstuff in a heat exchange device such as a tube in shell or plate-type heat exchanger. In contrast to indirect heat treatment systems, direct heat treatment systems bring the foodstuff into direct contact with a suitable heating medium such as steam. Although this direct contact with steam adds water to the foodstuff being treated, that added water may be separated from the treated foodstuff as desired.
Direct steam heat treatment systems can be divided generally into steam infusion systems and steam injection systems. In steam infusion systems, steam is directed through a steam inlet into a suitable steam chamber and the product to be treated is directed into the steam chamber through a separate product inlet, commonly a diffuser plate including a number of passages through which relatively fine streams of product may flow into the steam chamber. U.S. Pat. No. 4,591,463 describes examples of steam diffusion systems. In steam injection systems, a steam injector is used to inject steam into a stream of foodstuff flowing through a conduit to rapidly increase the temperature of the foodstuff to a desired treatment temperature. The added steam and product may then be held at an elevated temperature for a desired time by causing the mixture to flow through a hold tube. U.S. Pat. No. 2,022,420 provides an example of a steam injection system.
In both steam infusion and steam injection systems, the water added to the product during treatment may be removed from the product by applying a vacuum sufficient to vaporize the added water, and then drawing off the water vapor. This vaporization of added water also has the effect of rapidly decreasing the temperature of the now heat-treated product. In the case of steam infusion systems, the water and heated product are removed from the steam chamber and then directed to a vacuum chamber for applying the desired vacuum. In the case of steam injection systems, the mixture of heated product and added water is directed from the hold tube into a vacuum chamber where the added water is vaporized and may be drawn off along with any remaining steam.
Although direct steam injection systems are commonly used for heat treating foodstuffs such as milk and juices, problems remain which increase the cost of operating such systems. Perhaps the most persistent problem encountered in direct steam injection systems is the deposition of materials from the product, milk proteins in the case of milk treatment for example, on surfaces within the steam injector and downstream from the steam injector. Among other things, these deposits can reduce flow through the system and must be removed periodically to allow proper operation. This removal of deposits necessitates shutting down the treatment system and these shut downs increase operation costs and reduce productivity. In applications beyond dairy products, deposition may be so rapid that passages carrying the product to be treated become completely plugged in a very short period of time, a few seconds or a few minutes. The deposition problem thus prevents prior direct steam injection systems from being used for heat treating certain products, such as products including meat or egg proteins, especially raw (that is, uncooked) meat proteins in fibrous and other forms.
The problem of product deposition on surfaces of a direct steam injector is exacerbated by the configuration of product flow passages which are intended to facilitate quick and even heating of the product. In particular, direct steam injectors may be configured to produce a narrow stream of product to bring into contact with steam in the injector. In order to produce such a thin stream of product, a direct steam injector may cause the product to flow through a narrow flow passage, particularly a narrow annular flow passage, and steam may be brought into contact with the thin stream of product exiting the narrow flow passage. U.S. Pat. No. 3,988,112 shows an example of a steam injector in which the product to be treated is forced through a narrow annular flow path and steam is applied to the thin stream of product exiting the annular flow path. Although these injector configurations may be effective for allowing the product to be quickly brought to the desired treatment temperature, the narrow structures through which the product must flow are susceptible to rapid deposition of constituents from the product and are subject to plugging from deposited materials. The structure shown in U.S. Pat. No. 3,988,112 attempts to address the problem of product deposition on the injector surfaces downstream of the injection point by releasing a cold liquid along the surfaces containing the heated mixture. This patent also shows cooling surfaces of the injector downstream from the injection point by circulating a coolant through chambers formed in the walls of the injector downstream from the point where steam is injected into the product. While the surface washing and surface cooling in the injector downstream from the injection point may be effective to increase run times for some products, the techniques shown in U.S. Pat. No. 3,988,112 do not eliminate product deposition and may be entirely ineffective for some types of products. Also, the surface washing shown in U.S. Pat. No. 3,988,112 may lead to uneven heating in the product to be treated and may reduce the effectiveness of the heat treatment with regard to eliminating pathogens.
U.S. Patent Application Publication No. 2016/0143343 discloses a direct steam injector in which surfaces within the injector which come in contact with heated product such as milk are formed from polyether ether ketone, commonly referred to as PEEK, in an effort to reduce the tendency for product deposits to form on surfaces of the injector. PEEK is used in this prior injector not only for reducing the tendency for the formation of deposits and burning in the injector, but also for its resistance to cleaning agents and ability to withstand the temperatures encountered in the injector. However, the use of PEEK within the injector disclosed in U.S. Patent Application Publication No. 2016/0143343 does not eliminate product deposition and thus the injection system disclosed in this publication relies on a sensor arrangement which can be used to adjust flow paths within the injector to help ensure the desired level of heating in the product as deposits form on the injector surfaces.